Final Report on a Study of Fluid Inclusions in Core from Gibson Dome No

DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Final report on a study of fluid inclusions in core from Gibson Dome No. 1 bore, Paradox Basin, Utah by Edwin Roedderl U.S. Geological Survey Open-File Report 84-696 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (and stratigraphic nomenclature) 959, Reston, VA 22092 1984 Introduction Five small core samples of halite from the Department of Energy Gibson Dome No. 1 Bore (GD-1) were obtained for a preliminary study of the fluid inclusions present. These samples were from evaporite cycle 6 of the Paradox Member of the Hermosa Formation. The aim of this investi gation was to see what information such a study might provide as to the geologic behavior of such fluids if these salt beds were used for a nuclear waste repository, as well as the geologic processes involved in the formation and subsequent history of these saline beds. Samples studied The studied samples consisted of one-fourth of the 4-inch diameter core from the following depth intervals, measured in feet from the Kelley bushing: 3,148.9 - 3,149.1 3,185.0 - 3,185.2 3,239.1 - 3,293.3 3,279.6 - 3,279.8 3,321.5 - 3,321.7 These samples were selected in consultation with R.J. Hite, USGS, and were released to the USGS per letter dated August 3, 1982, PXX-82-281 , from N.A. Frazier, Project Manager, Paradox Basin Exploration Office, to Mr. Fred Conwell, Woodward-Clyde Consultants. Sample preparation The selected portions were cut from the core with a diamond cutoff wheel and Almag cutting oil, and then cleaned of cutting oil by washing with trichloroethane. One or more doubly polished plates, mounted with cold-setting epoxy, were then cut from each sample, perpendicular to the bedding and across visible color or textural variations. Grinding and polishing was done with alcohol as a lubricant and coolant, and care was used to avoid any heating. Orientation of these plates relative to gravity was maintained, in case geopetal textures might be present. Petrographic examination General features. Although the five samples differed in appearance, and were scattered through a 172-foot interval, relatively few differences could be recognized between the five samples in this reconnaissance and hence they will be discussed here as a group. Primary inclusions in unrecrystallized salt. Only a very small percentage of the salt crystals in these samples shows good evidence of primary crystallization features from the original crystal growth in a salt basin. The main evidence for primary crystallization is the presence of chevron growth features, outlined by alternating zones of primary fluid inclusion-rich and inclusion-free salt (Roedder, 1984a). The inclusions in most such salt are very small and densely packed (Fig. 1). The change from inclusion-rich to clear salt can be abrupt (Fig. 1 insert). Few inclusions are >10 urn, and many are <1 jim. As is common in such chevron salt, some periods of salt dissolution occurred during deposition of the original salt beds in the Paradox Basin, leaving curving surfaces cutting across the growth banding. Subsequent (presumably slower) growth on these curved surfaces can form a clear extension of the original crystal, in crystallographic continuity (Fig. 2). The orientation of the chevrons can provide geopetal information, as the apex of the chevron points stratigraphically upward (Fig. 3); such geopetal features were rare in these particular samples. Practically all inclusions making up the chevrons in the GD-1 samples contain a single large daughter crystal that has crystallized out of the fluid after trapping. The only exceptions are inclusions <~3 \w> presum ably this difference is merely a result of nucleation problems (Roedder, 1971). The high birefringence of this daughter phase makes these crystals stand out between partly crossed polars (Fig. 4). Occasional larger, pri mary inclusions within chevron growth (e.g., "X" in Fig. 3) permit esti mates of the volume percentages of the phases in such inclusions. These inclusions generally contain ~25 vol.% of the daughter phase (e.g., Fig. 5). In other inclusions another different birefringent phase is also present, as very irregularly distributed separate crystals that may range from 0.5 vol.% to >30 vol.fc of the individual inclusion. Only rarely do these primary inclusions in unrecrystallized salt show a tiny vapor bubble; estimated homogenization temperatures for these bubbles would be ~ 40°C. The large daughter crystals have optical and crystallographic proper ties, as best can be measured within an inclusion, that match those of carnallite, KMgC^'Sf^O, and all such crystals are assumed to be carnallite in the balance of this report. These properties include: generally par allel extinction, birefringence ~0.03, lower index shows low relief against the fluid phase, (pseudo)hexagonal habit (Figs. 6 and 7), frequently with multiple (pseudo)hexagonal pyramids (Fig. 8). These features, plus their high solubility in water, their very high thermal coefficient of solubility (see below), and the known presence of K and Mg, and of carnallite as a rock-forming mineral this section of the Paradox Basin salt beds, make the identification of the daughter crystals as carnallite fairly unambiguous. However, to be certain, several attempts were made to extract these crystals, using the crushing stage (Roedder, 1970). The extracted single crystals were mounted on fibers and X-ray powder diffraction patterns obtained using the Gandolfi camera technique of Zolensky and Bodnar (1982). The five lines obtained agreed well with the five strongest lines of carnallite. The density of the other phase or phases that are present in such highly variable amounts is unknown, but the optical properties fit anhydrite, and anhydrite (verified by X-ray diffraction) is also present as crystals embedded in nearby salt. The variable abundance of these crystals in the fluid inclusions, and the lack of any change in them during heating studies (see below) makes it apparent that they are not daughter crystals, but accidentally-trapped solid inclusions phases that were present during the growth of the host chevron salt. Primary inclusions in recrystallized salt. When a mineral recrystal- Hzes in the presence of a fluid phase, Inclusions of that fluid can become trapped in the new crystals as primary inclusions in recrystallized host. The time of trapping of such crystals can be immediately after the original crystallization of the host crystal in the salt pan, or during subsequent diagenesis and recrystallization, perhaps millions of years later (Roedder, 1984a). Much of the halite in these samples is free of chevron growth and appears to have recrystallized. Although the resulting crystals make up a tight fabric of low porosity, they are generally smaller (3-4 mm) than the recrystalllzed salt at the WIPP site (Roedder and Belkin, 1979). Most of the larger fluid inclusions in the Gibson Dome samples appear to be in such clear, recrystallized salt. During recrystallization, small amounts of the brine present at the time tend to be trapped at the inter face between various solid inclusions and the host halite (Fig. 9). In other cases, a solid crystal may be trapped within a fluid inclusion (Fig. 10), or a large number of solid crystals may be trapped (Fiq. 11). In the last example, however, a new large, true daughter crystal (x) also formed on cooling from the temperature of trapping to room temperature. The primary inclusions in recrystallized salt also have large daughter crystals of carnallite, like those in the chevron salt, and sometimes a small shrinkage bubble (~ 0.1 vol.%; Figs. 11 and 12). The volume percent of carnallite is approximately the same in both chevron and recrystallized salt (~ 25 vol.%). Rarely a small ragged mass of opaque material is present, presumably organic matter. Some small inclusions of clear to brownish fluids that were insoluble in water (presumably hydrocarbons) were found in halite in this and other parts of the core. In addition to the numerous blades of anhydrite, carnallite, etc., that are embedded in the halite host, some highly birefringent, very thin square plates, with inclined extinction, were found that appeared to be in some salt grains (Fig. 13). These, however, turned out to be artifacts, formed on the lower polished surface of the salt sample before it was cemented to the glass slide. Their nature is unknown, and they are men tioned here only as a caveat to other workers. Secondary inclusions. Very few recognizably secondary inclusions were found in these samples, but in view of the speed with which inclusions in salt can change shape and position, many other inclusions may actually be secondary in origin. The only inclusions that can be assumed to be secondary are those arranged along apparent healed fractures, rather than on growth planes. Since salt cleaves parallel to the common growth plane (100), ambiguity remains in many examples. One possible candidate for secondary origin (Fig. 14) consists of a curving group of liquid inclusions, each containing only liquid and a very small bubble, that occurs in salt in which the primary inclusions all contain large daughter crystals of carnal!ite. Another possible example is found in some planes of what apparently were originally high pressure gas inclusions, now decrepitated (Fig. 15). Overall estimate of free water contents. As in most salt (Roedder and Bassett, 1981), water is present in salt bed 6 in a variety of forms (Fig. 16). It is present as bound water in hydrous minerals such as carnallite, as free water in intracrystalline fluid inclusions within single crystals, and as intercrystalline films and fluid inclusions along grain boundaries between crystals. Since these samples have been exposed to air, the intercrystalline, grain-boundary fluid has long since evapor ated, leaving air inclusions that generally appear black from total reflection (Fig.

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